The present invention relates to a method of forming aluminum coating layers and more particularly, to a method of forming aluminum coating layers having corrosion resistance at high temperatures and resistance against oxidation in a halogen atmosphere on surfaces of ferrous base alloy workpiece.
As is well known, exhaust gases discharged from motor vehicles contain halogen gases, halogen compounds and lead compounds, for example, Cl.sub.2, Br.sub.2, PbCl.sub.2, C.sub.2 H.sub.2 Cl.sub.2, C.sub.2 H.sub.2 Br.sub.2, etc. besides unburnt noxious gases including carbon monoxide, hydrocarbon and the like, and components or parts made of ferrous base alloy material for exhaust system of the motor vehicles, for example, heat exchangers, air ducts, containers, etc. tend to be subjected to corrosion by the noxious compounds as described above. Moreover, halogen compounds (e.g. salt) employed for preventing freezing during cold seasons are liable to enter these components of ferrous base alloy material, which are then corroded by the atmosphere containing halogen gas produced when the halogen compounds are decomposed at high temperatures.
Conventionally, it has been a general practice to employ components or parts produced by forming ceramic layers on the surfaces of ferrous base alloy materials as corrosion resistant members against atmospheres containing halogen at high temperatures, but when the known corrosion resistant members as described above are used in places which are subjected to repeated heating and cooling as in the motor vehicle components, the ceramic layers tend to be peeled off in a short period of time due to difference in thermal expansion coefficients between the ferrous base alloy materials and ceramic coating layers without sufficient durability, thus not being suitable for practical application.
In order to overcome the disadvantages as described in the foregoing, the present inventors have previously proposed in U.S. Pat. Nos. 3,907,611, 3,941,569 and 4,079,157 a method of manufacturing components having high temperature corrosion resistance in a halogen atmosphere which includes the steps of dipping the workpiece of ferrous base alloy material such as stainless steel into a molten bath of Al or Al alloy to apply melt-plating on the surfaces of the workpiece, subjecting the workpiece thus prepared to heat treatment at high temperatures of 700.degree. to 950.degree. C. to form thereon the Al compound layer mainly composed of Al-Fe compound, and further subjecting the workpiece, if necessary, to heat treatment at temperature range of 950.degree. to 1350.degree. C. to form thereon the alloy layer having Fe and Al as main components.
The known method as described above, however, still has a disadvantage in that the expensive facilities are required for melting Al or Al alloy, while it is rather difficult to precisely control the thickness of the plated layer to be formed on the workpiece of ferrous base alloy material.
Meanwhile, there has also been conventionally proposed, for example, by Japanese Laid Open Patent Application Tokkaisho 52-108344, a method of manufacturing corrosion resistant members for exhaust gas treating apparatuses which includes the steps of covering the surface of the ferrous base alloy workpiece with a suspension containing water ceramic binder, aluminum particles, phosphoric acid, and chromic acid with subsequent heating and drying of the workpiece, and subjecting the workpiece thus treated to heating in the exhaust gas from engines containing residual oxygen to form a diffusion layer mainly composed of alumina on the surface of the workpiece for imparting thereto high temperature corrosion resistance. Since the corrosion resistant member obtained by the above known method, however, has its surface covered by a porous ceramic layer, with the aluminum almost oxidized to form the alumina diffusion layer, the aluminum coating layer required is not obtainable, and even when the workpiece is further heated over 700.degree. C., it is not possible to form the Fe-Al compound layer or alloy layer mainly composed of Al and Fe, thus the resultant corrosion resistant member thus produced being considered to be comparatively inferior in the corrosion resistance at high temperatures.